To store frozen food over a period of many days, particularly in connection with warehousing and large retail store operations, it is common to provide a cold storage room. A cold storage room, which may be of any desired size, is well insulated, has a refrigerating system associated with it, and at least one entrance way with a door. Since it is important that the operator know the temperature which is being maintained in the cold storage room, the cold storage rooms are provided with a thermometer. Typically, the themometers are electrically operated, having a thermoelement and an indicator.
In their perferred forms, the thermoelement comprises a selected theromcouple and the indicator is preferably a common numerical LED digital display indicator with changable numbers. The thermometer is supplied with current, typically utilizing line current at approximately 115-120 volts A.C., connected to the primary winding of a transformer, the secondary winding of which generates a lower voltage, such as nine volts.
It has been recognized that power failure of the current to the main supply lines for the refrigeration system occurs, and equipment has been provided either to give alarms, or to connect alternate power sources. For example, Jeppesen U.S. Pat. No. 2,740,953 provides a system in which line current is supplied to refrigeration apparatus, and is caused to pass through a relay. When the line current stops, as due to power failure, the relay becomes inactive, causing the closing of a switch which thereby completes a circuit which includes a back-up battery and an indicator, so that the indicator, such as a bell and a light are energized to give an alarm that there has been a power failure. Day, III et al., U.S. Pat. No. 4,387,368 discloses a water chilling system in which a back-up battery is maintained charged by a battery charger operated from the main power line. Upon failure of line current, the battery is placed in the circuit, to provide an alternate source of power to operate the water chilling equipment.
Wootton U.S. Pat. No. 3,689,888 provides an alarm system including a battery and a battery charging circuit, the battery being placed in operation when the power supply fails. A sensing circuit is provided for sensing a number of failed situations including power failure, refrigeration loss, etc.
Donovan U.S. Pat. No. 3,753,259 discloses a warning system for use with a refrigerated food case, the system including a battery used to provide back-up power in order to give a signal upon failure of the main power or current input.
Messmann et al., U.S. Pat. No. 3,959,787 provides a system for generating an alarm if there is an improperly high temperature within a refrigerator. The system includes a thermostat within the refrigerator which is connected to an alarm unit on the outside of the refrigerator, the alarm unit including a battery and a lamp, the circuitry providing for the tripping of a solenoid, thereby closing the battery and lamp circuit, upon the occurence of an improperly high temperature within the refrigerator.
In all of the above disclosures, the battery in the back-up power circuit or in the alarm circuit, once energized, remains energized, and is thereby continuously depleted.